Stormwater is rainwater plus particulate debris and dissolved materials that the rainwater carries along with it. In urban areas, rain that falls on the roofs of houses, collects on paved areas like driveways, roads and sidewalks is typically diverted through a system of pipes that is separate from the sewage system. Unlike sewage, stormwater was historically not treated, but flowed directly from streets and gutters into rivers, lakes and oceans.
Stormwater can be a form of diffuse or non-point source pollution. It can entrain pollutants, such as garbage, sediment, organic matter, heavy metals, and organic toxins, and flush them into receiving water bodies. As a consequence, natural bodies of water that receive stormwater may also receive pollutants capable of irreparable environmental harm.
The amount of stormwater pollution entering into such receiving bodies of water is related to the degree of urbanization in the surrounding area and the nature of the surrounding activities. Urbanization results in the covering of land with low-permeability structures, such as roadways, parking lots, and rooftops, which both generate large volumes of stormwater and accumulate pollutants. Since these types of surfaces do not allow rainfall to infiltrate, they allow the accumulated pollutants to be washed into stormwater drainage systems.
One known stormwater drainage system is a dry well. Dry wells may be formed by drilling or digging a vertical hole into the ground, for example, 10 to 30 or more feet deep, installing a structure or pipe with perforations in the wall of the structure or pipe and filling the hole around it with gravel. The stormwater flowing into this structure or pipe migrates out through the perforations and is returned to the ground after passing through the surrounding gravel.
A filtration system in the form of a buffer tank has been proposed to remove sediment and pollutants from the water prior to entering the dry well. Filters are used to remove the sediment and pollutants from the water as it passes through the buffer tank on its way to the dry well drain pipe.